In the context of installations in great depths, undersea pipes and sets of undersea coaxial pipes are assembled together on land to constitute elements of unit length, lying in the range 10 m to 100 m, depending on the loading capacity of the laying system. They are then taken to sea in that form on a pipe-laying ship. While being laid, the unit lengths of the various coaxial pipe assembly elements are connected to one another on board the ship and they are progressively laid at sea. It is therefore important for such connections to be capable of being made quickly and easily.
For this purpose, steel junction parts or connection forgings are used that are assembled to the ends of said coaxial pipe assembly elements that are to be assembled together. The junction forging at the downstream end of a first coaxial pipe assembly element that has not yet been assembled is connected to the junction part at the upstream free end of a second coaxial pipe assembly element that has already been assembled at its downstream end.
More particularly, a said junction forging at the end of a coaxial pipe element includes, at each end, two branches forming bodies of revolution, namely an outer branch and an inner branch together forming a fork that defines a said annular space, thereby constituting a first fork having its free cylindrical ends assembled to the cylindrical ends respectively of the outer and inner pipes, and a second fork having its ends assembled to the facing ends of the junction forging of another coaxial pipe element.
Such junction forgings generally seek to improve the strength of pipes that are subjected to high levels of bending during laying, in particular in the zones where two successive unit lengths or successive strings are connected together; and more particularly, in a bottom-to-surface connection, to give them very great resistance to fatigue throughout the lifetime of an installation. Coaxial pipes and junction forgings of this type have already been described, in particular in FR 2 873 427.
Prior assembly of the various pipe portions on board the ship, followed by progressive laying of the pipe on the sea bottom by assembling strings together and then immersing the pipe progressively as the strings are assembled together, is conventionally performed, in particular for laying undersea pipes in depths of more than 1000 m, or indeed more than 2000 m, from a floating support or ship that is fitted with an optionally hinged tower that includes handling means or gripper means, by a so-called J-lay method in which the suspended undersea pipe adopts a curved configuration between its point connected at the surface to the ship and its point in contact with the sea bottom, said shape corresponding to a mathematical curve known as a catenary, i.e. a curve that presents a radius of curvature that increases regularly from the bottom up to the surface.
This kind of laying is referred to as “J-laying” because the shape of the portion of pipe that is in suspension between the ship and the point of contact on the sea bottom is somewhat incorrectly said to be “J-shaped”. J-laying is described in numerous patents, in particular the following patents in the name of the Applicant: FR 2 792 991, FR 2 793 540, and FR 2 801 088, amongst others.
Those patents describe a pipe-laying ship fitted with a J-lay tower that serves to perform the following operations:
holding stably the emerging top end of the portion of pipe that has already been assembled and that is immersed in suspension;
lowering the new pipe portion or string, so as to present the bottom end of said new pipe portion or string to said emerging top end of said suspended pipe portion;
firmly holding the ends of the pipe portions that are to be assembled together by welding so that said welding can be performed without risk of damage due to the various movements of the ship and of the immersed pipe portion that is in suspension down to the sea bottom; and finally
welding said pipe portions together.
In such J-laying, a difficult lies in the fact that all of the above-mentioned operations need to be performed at a single location situated on the deck of the ship, i.e. close to the bottom end of the tower, and furthermore all of the operations need to be performed in a minimum amount of time because of the extremely high hourly cost of running the installing ship.
The pipe being assembled is generally held in the bottom portion of the J-lay tower by an external system of clamps.
The use of steel marking-wedge clamps for holding stationary the suspended pipe portion presents the drawback of leaving traces on the outside of the pipe, and above all of damaging its anti-corrosion coating. Numerous other types of clamp have been developed, essentially based on hinged toggle systems or on systems that are locked by cams, with the movements of engaging and disengaging the clamps often being performed by means of hydraulic actuators.
Such clamps are well adapted when loads are small, however in very great depths, i.e. 2500 m, the weight of a pipe may exceed 500 (metric) tonnes (t) and it is necessary to guarantee an extremely high level of safety regardless of the type of pipe during the end-to-end connection stages that may last for 15 minutes to 45 minutes per cycle, or indeed in the event of operations being interrupted for various reasons such as technical incidents or bad weather, which can last for several days. If there is no need to preserve the outer coating of the pipe intact, then marking-wedge clamps as described above perform their function of retaining the pipe perfectly, even if the pipe presents irregularities of diameter, but if said coating is fragile, then it is not possible to use that type of clamp.
Numerous solutions have been developed to avoid damaging the outsides of pipes, however they are not suitable for extreme loads, since they present the drawback of not providing positive safety in the event of sliding phenomena starting, which phenomena run the risk of leading to severe accidents, not only for the equipment, but also for personnel.
In patent FR 2 801 088 in the name of the Applicant, a clamp is described that operates on the friction principle, thereby implying the use of actuators that are disposed substantially perpendicularly to the outside surface of the pipe and that are used for gripping it.
As the laying depth increases, the linear weight of a pipe generally also increases, since pipes need to be capable of withstanding implosion, given that they are laid while empty, i.e. the pressure inside the pipe is substantially atmospheric pressure, whereas the pressure outside the pipe is substantially 1000 bars, i.e. 10 megapascals (MPa) for every 1000 m of depth. Thus, not only does the suspended length increase with depth of water, but the thickness of said pipe also increases, thereby increasing its unit weight. Laying tensions may then be as great as or exceed 1200 t to 1500 t, or even more, and the friction clamps of FR 2 801 088 then present dimensions and complexity that are excessive in order to achieve a very high level of reliability in the operation of laying installations. In addition, the very large dimensions of such devices give rise to considerably increased purchase for currents and swell, thereby requiring greater power for dynamically positioning the ship whose position needs to be maintained very accurately.
U.S. Pat. No. 6,273,643 describes a system for holding and retaining the pipe in a tower, in which the top end of the pipe is rigidly fastened to a platform that is secured to the bottom end of the tower, and the inclination or the curvature of the pipe beneath the rigid fastening of its top end is controlled by a tubular structure device having a larger number of rollers so as to hold the pipe substantially on the axis of the tubular structure.
An installation of that type might generate non-uniform stresses at the pipe-tower junction, which requires safety margins to be provided concerning the mechanical strength of the rigid junction parts, and thus leading to junction parts that are expensive.
The problem posed is thus that of holding an undersea pipe under tension while it is being laid by means of an improved method and device for retaining and holding the pipe, that are simpler and less expensive to implement while nevertheless retaining a maximum level of reliability.